The infection of HHV-6 occurs very early in childhood (under one year) and virus remains for life in a latent state. Using the peripheral mononuclear cells in culture from HIV-1-positive AIDS, chronic fatigue syndrome (CFS), systemic lupus erythematosus (SLE) patients and bone marrow transplant recipients, a high frequency of virus activation (40%) compared to normal donors (12%) was detected indicating active virus replication. HHV-6 was found to induce IL-1b and TNF-a in human peripheral blood mononuclear cells. In contradistinction, HHV-6 has no effect on IL-6 induction. Cytokine induction appeared to be mediated by early viral protein. Cytokine production by HHV-6 in cells suggests how HHV-6 may contribute to disease manifestation. HHV-6 antigen-positive cells were detected by in situ hybridization and by monoclonal antibody assays in Hodgkins disease, (HD), AIDS, SLE and Sjogren's syndrome patients' cells. The virus DNA-positive cells expressed CD38+, CD4+, and to lower degree CD8+ and CD21+ cell receptors. Using ELISA and HHV-6 early protein (P41/38), sera from African Burkitt's lymphoma (ABL), HD and American Burkitt's lymphoma (AmBL) showed high frequency of antibody to P41/38 (ABL, 96.5%; HD, 62.5%) compared to normals (12%), whereas ELISA to HHV-6 late antigen (gp110) failed to detect these differences. These findings indicated active HHV-6 replication and suggested a possible role of this virus in the pathogenesis of these malignancies. Thus, this assay may be a good marker for active and chronic HHV-6 infections.